Average ion moments in the plasma sheet boundary layer

Using four months of tail data obtained by the three‐dimensional plasma instrument on board the AMPTE/IRM satellite in 1986, we have done a statistical survey on the behavior of ion moments and related plasma parameters in the plasma sheet boundary layer and the immediately adjacent outer central plasma sheet. The more than 40,000 spin averages of ion density, ion bulk velocity, ion temperature, ion temperature anisotropy and ion β were analyzed with respect to differences between their values in plasma sheet boundary layer and adjacent central plasma sheet. We also looked into their dependence on magnetic activity, and possible interrelations among them. As a primary result, we find that the plasma sheet boundary layer is not just a transition region where plasma parameters are gradually changing from plasma sheet to lobe values, but is indeed a unique layer where crucial parameters like ion density and ion β remain on nearly constant levels which are different from those in the central plasma sheet. Accordingly, there is a sharp gradient in ion density and ion β at the interface between the plasma sheet boundary layer and the central plasma sheet. A second important finding is that the average flow velocities in the plasma sheet boundary layer are surprisingly low, typically below 100 km/s. High‐speed flows do occur quite often and under all levels of magnetic activity, but in bursts of often less than one minute duration with intermittent intervals of nearly stagnant plasma. Their rate of occurrence is higher during high AE and, in particular, during plasma sheet expansions. All high‐speed flows are field‐aligned and directed toward the Earth.